Repair ancient race condition that allowed a transaction
to be seen as committed for some purposes (eg SELECT FOR
UPDATE) slightly sooner than for other purposes

This is an extremely serious bug since it could lead to
apparent data inconsistencies being briefly visible to
applications.

Repair race condition between relation extension and
VACUUM

This could theoretically have caused loss of a page's
worth of freshly-inserted data, although the scenario seems
of very low probability. There are no known cases of it
having caused more than an Assert failure.